1. Field of the Invention
The present invention is related to the field of data transmission over networks, and more specifically to devices and methods for adjusting a transmission rate of streaming media depending on available bandwidth.
2. Description of the Related Art
Networks are increasingly used to transmit data. The data can be for various media, such as audio or video, and is usually organized in groups that are called frames. When such data is transmitted continuously, it is also called streaming media. The transmission can either be for live, or for canned media. Either way, the transmission has to be in real-time, or in delayed real-time.
A fundamental requirement for streaming media is that it has to be played at a constant rate. Packet networks, however, may transmit data asynchronously. Therefore, for streaming media to be transmitted over packet networks, strict timing requirements have to be met. Furthermore, where there are two related but separate streams of media (such as audio and video), they have to be time-synchronized in order to achieve high-quality media play out.
A network channel should be used that has a higher bandwidth than what is required by the streaming media. The network, however, is occasionally subject to congestion, which constricts the bandwidth of the channel. This causes play out of streaming media to exhibit the problem of network jitter. This means that what is played out might be momentarily discontinued.
Referring to FIG. 1, the problem of network jitter has been addressed in the art by introducing a de-jitter buffer at the receiving end. A server or proxy 110 receives data from a source, and stores it in a transmit buffer 114. The proxy 110 then transmits the stored data through a network 150 to a client or receiver 160. Typically, the client 160 includes a fixed size de-jitter receive buffer 162.
The transmit buffer 114 transmits data at the regular rate. It does not transmit any faster, because it if did, it would overflow the buffer 162. The buffer 162 first fills up to its size with data, and then starts playing out at the same rate as it is being filled. Momentary constrictions in bandwidth affect only the fullness of the buffer, not the play out, therefore diminishing network jitter.
The buffer 162 does not work well where there are high-bit rate media streams, which are transmitted through heavily congested networks. In these cases network jitter still occurs, notwithstanding the buffer 162.
Referring to FIG. 2, the phenomenon of network jitter is now described in more detail. Various quantities are plotted against a time axis, which is shown as a line 210.
A solid, piece-wise linear graph 220 represents the time evolution of the available bandwidth of network 150. Notice that the graph 250 has a nominal value BI, but which is not always the case. Indeed, for a heavily congested network, the graph 250 has two main bandwidth constrictions, which are represented by “wells” A and B. These are instances where the available network bandwidth drops to values below BI. Well A is deeper than well B, but does not last as long.
A dashed line 230 represents the instantaneous rate of receiving data through the network. In FIG. 2, the line 230 starts at a value fy, which is the steady state, assuming no prior interruptions in the transmission. The value fy is also the streaming media play out rate. That is how fast the data stream is being transmitted from the proxy 110. In addition, line 230 is always less than the instantaneous value of graph 220. For example, the line 230 tracks the graph 220, as it decreases in well A and well B. Immediately after the wells, however, the dashed line 230 increases, as formerly bottlenecked data is released from the network. After a long, uninterrupted regular receiving time, line 230 settles back at a value fy.
A line 240 represents the instantaneous fullness of the buffer 162. In FIG. 2, initially the buffer 162 is at its nominal size fy, after a long time of network congestion.
The fullness of receiving buffer 162 remains at approximately the same level, until the first dip occurs (well A). Line 240 follows with a corresponding well A′, and then recovers. In more mathematical terms, line 240 is determined from a time integral of the difference between line 230 minus the value fy. For example, if well A did not reach below fy, line 240 would not exhibit well A′ at all.
Similarly, due to well B, line 240 exhibits a well B′, and then recovers. Well B′, however, reaches zero, with minor blips 246. This corresponds to a situation of a buffer under flow. Data is still arriving (line 230 is above zero), but slower than it is expected to be played (line 230 is less than value fy). Between time points 242, 244, play out is interrupted. This interruption is called network jitter.
As will be appreciated, network jitter is a disruption in play out of the streaming media. If it is video, then the image stops, or flashes intermittently. If it is audio, then the sound is interrupted. In both cases, network jitter is annoying to the end user.